1. Field of Invention
This invention relates to rotary kilns, and in particular to improving the delivery of preheated charge material from a rotary kiln to a liquefier in a multi-stage melting process. The invention is particularly applicable to preheating batch material in a glass melting operation, but is applicable to other processes that thermally convert a generally flowable, essentially solid state feed material into a molten fluid.
2a. Technical Considerations
It has long been recognized that exhaust gas from a combustion heated glass melting furnace or other melting process furnace contains large amounts of thermal energy that can be recovered to improve the overall efficiency of the process. Conventionally, regenerators and recuperators have been employed to recover heat from melting process furnaces by preheating combustion air, but their efficiency is less than desired. Instead of preheating combustion air by way of regenerators or recuperators, it has been proposed to recover waste heat by means of preheating the feed material.
The amount of waste heat from a melting furnace useable in preheating feed material is generally limited by a specific batch condition within a preheating vessel that is sought to be avoided. For example, typical batches of float glass contain sand, soda ash, limestone and dolomite, as well as other materials. When the batch is preheated in a rotary kiln and reaches a certain temperature range, the batch material will begin to agglomerate and stick to the kiln prior to its discharge into the melting furnace, resulting in clogging of the rotary kiln and the batch transfer arrangement. To avoid this condition, the components of the feed can be preheated separately and combined directly at the furnace. Such an arrangement requires additional feeders and metering devices to ensure the correct material mix in the batch. In the alternative, if all the batch constituents are combined and preheated in the same vessel, the temperature of the batch material must be maintained below the temperature that will cause agglomeration of the batch.
In U.S. Pat. No. 4,381,934 to Kunkle and Matesa, there is disclosed an intensified batch liquefying process in which large volumes of batch are efficiently liquefied in a relatively small space. This type of process, particularly when using intensified heat sources, produces relatively small volumes of high temperature exhaust gas. It would be desirable to recover the heat from the exhaust gas to further improve the efficiency of such a process and in particular, to recover the heat directly into the batch feed stream while maintaining the preheating temperature of the batch feed below the temperature that results in agglomeration and/or sticking of the batch material.
2b. Patents of Interest
U.S. Pat. Nos. 118,674 to Baynton, 939,817 to Edison; 1,510,956 to Perkins; 1,800,247 to Buckbee, 1,869,237 to Bruhn; and 3,682,453 to Powell each teach the use of cooling jackets on rotary kilns to control the temperature of the processed product within the kilns prior to discharge and/or reduce high thermal stresses and the accompanying thermal degradation of the kiln. In Baynton and Edison, the exterior of the kiln is sprayed with a water bath to cool the outer shell. In Perkins, the exterior walls of the kiln are provided with buckets to pick up water from a reservoir positioned below the kiln. As the kiln rotates the buckets pour water over the outer skin of the kiln. In Buckbee, 1ongitudinally extending passages positioned around the perimeter of the kiln are air cooled by stationary blowers. In Bruhn, a water cooled jacket is used to reduce the temperature of the rotary furnace at its sintering zone. In Powell, longitudinal passages are supplied with cooling air to protect the discharge end of the kiln from excessive heat.
U.S. Pat. No. 1,861,266 to Forse et al. teaches a method of controlling the rate of cooling in a kiln after it has been subjected to the maximum temperature of the kiln. Cooling air is blown into air ducts positioned in the refractory walls and ceiling.
U.S. Pat. No. 2,363,390 to Buehl teaches a method of cooling sponge iron before it is discharged from a rotary kiln into the air to minimize oxidation. The sponge iron is deposited from the kiln into a pipe coiled around the kiln outlet. The pipe with the hot sponge iron is cooled by a water spray while the pipe is partially submerged in a water reservoir.
U.S. Pat. No. 3,703,277 to Bossard teaches the use of longitudinally extending closed boiler tubes positioned generally in the direction of the kiln axis of rotation at the kiln outlet. Liquid in the hot region of the tubes is acted upon by the heat from the kiln and is evaporated. The vapor condenses in the cold region of the tube giving up its heat to the surrounding atmosphere.
U.S. Pat. No. 4,102,530 to Hawkes et al. teaches an annular cooling coil surrounding the discharge end of a rotary furnace. Cooling fluid is continuously circulated from a delivery trough through the coil to a collecting trough.
U.S. Pat. No. 4,391,583 to Serbent et al. teaches a method of preventing crusting of charge components on the refractory surface of a rotary kiln. Heat is reduced in several sections of the kiln so that the temperature of the inside surface of the kiln that is submerged under the charge is always 122.degree. F. (50.degree. C.) or less cooler than the charge. Cooling of the inside of the surface is effected by sprinkling water on sections of the kiln's outer shell, or by means of cooling pipes which are installed in the refractory lining of the kiln. The temperature of the inside surface of the kiln is measured at several points and the dissipation of the heat along the length of the kiln is adjusted accordingly.